Small optimizations in 'string.gsub'

Avoid creating extra strings when possible:

- avoid creating new resulting string when subject was not modified
(instead, return the subject itself);

- avoid creating strings representing the captured substrings when
handling replacements like '%1' (instead, add the substring directly
to the buffer).

2 files changed, 31 insertions, 1 deletions

diff --git a/testes/gc.lua b/testes/gc.lua
index 91e78a48..6d24e0d8 100644
--- a/testes/gc.lua
+++ b/testes/gc.lua
@@ -113,7 +113,7 @@ do
   contCreate = 0
   while contCreate <= limit do
     a = contCreate .. "b";
-    a = string.gsub(a, '(%d%d*)', string.upper)
+    a = string.gsub(a, '(%d%d*)', "%1 %1")
     a = "a"
     contCreate = contCreate+1
   end
diff --git a/testes/pm.lua b/testes/pm.lua
index 8cc8772e..4d87fad2 100644
--- a/testes/pm.lua
+++ b/testes/pm.lua
@@ -387,5 +387,35 @@ assert(string.match("abc\0\0\0", "%\0%\0?") == "\0\0")
 assert(string.find("abc\0\0","\0.") == 4)
 assert(string.find("abcx\0\0abc\0abc","x\0\0abc\0a.") == 4)
 
+
+do   -- test reuse of original string in gsub
+  local s = string.rep("a", 100)
+  local r = string.gsub(s, "b", "c")   -- no match
+  assert(string.format("%p", s) == string.format("%p", r))
+
+  r = string.gsub(s, ".", {x = "y"})   -- no substitutions
+  assert(string.format("%p", s) == string.format("%p", r))
+
+  local count = 0
+  r = string.gsub(s, ".", function (x)
+                            assert(x == "a")
+                            count = count + 1
+                            return nil    -- no substitution
+                          end)
+  r = string.gsub(r, ".", {b = 'x'})   -- "a" is not a key; no subst.
+  assert(count == 100)
+  assert(string.format("%p", s) == string.format("%p", r))
+
+  count = 0
+  r = string.gsub(s, ".", function (x)
+                            assert(x == "a")
+                            count = count + 1
+                            return x    -- substitution...
+                          end)
+  assert(count == 100)
+  -- no reuse in this case
+  assert(r == s and string.format("%p", s) ~= string.format("%p", r))
+end
+
 print('OK')